hoistop, I think this is a good question. The AAIB report on page 5 states that this nut was "found to have a torque load significantly higher than the required assembly value".
In addition, on page 7, paragraph 2 "the increased torque load on the castellated nut that remained on the spider end of the shaft is consistent with rotation of the tail rotor actuator control shaft"
Are they saying the jammed bearing would tighten the nut? If the rotor is turning anti-clockwise, the bearing jams and the thread is clockwise it does not seem to make sense. Or does it mean it could have tightened on impact?